Catalytic oxidative dehydrogenation of paraffins

ABSTRACT

Method for the oxidative dehydrogenation of paraffins to produce olefins by contacting the paraffins with molecular oxygen in the presence of molten alkali metal hydroxides, aluminum, and a soluble transition metal oxyanion included in the molten alkali metal hydroxide.

United States Patent Tomezsko 1 Oct. 10,1972

[541 CATALYTIC OXIDATIVE DEHYDROGENATION OF PARAFFINS [72] Inventor: Edward S. J. Tomemko, Media, Pa.

[73] Assignee: Atlantic Richfield Company, New

York, NY.

[22] Filed: March 16, 1971 21 Appl. No.2 124,979

3,586,733 6/1971 Connoretal ..260/683.3

Primary Examiner-Curtis R. Davis Attorney-John D. Peterson, John J. McCormack and Michael B. Fein ABSTRACT Method for the oxidative dehydrogenation of paraffins to produce olefins by contacting the paraffins with molecular oxygen in the presence of molten alkali metal hydroxides, aluminum, and a soluble transition metal oxyanion included in the molten alkali metal hydroxide.

7 Claims, No Drawings CATALYTIC OXIDATIVE DEHYDROGENATION OF PARAFFINS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a novel catalytic method for the dehydrogenation of paraffins.

2. Prior Art U.S. Ser. No. 844,608, filed July 24, 1969, now US. Pat. No. 3,586,733, discloses a method for the oxidative dehydrogenation of paraffins to produce olefins by contacting the paraffins with molecular oxygen in the presence of molten alkali metal hydroxides, and aluminum metal or activated alumina.

SUMMARY OF THE INVENTION In accordance with this invention a gaseous mixture of a paraffin (alkane) having from two to 12 carbon atoms, preferably from three to six carbon atoms, together with molecular oxygen is passed through a bed of molten alkali metal hydroxide containing a soluble transition metal oxyanion dissolved therein and aluminum metal, thereby dehydrogenating the paraffin to the corresponding olefin. It is an object of this invention to provide an improved method for the oxidative dehydrogenation of paraffins.

It is another object of this invention to provide a method for the oxidative dehydrogenation of paraffins to olefins utilizing molecular oxygen. It is a further object of this invention to provide a method for the oxidative dehydrogenation of paralfins to olefins using molecular oxygen, molten alkali metal hydroxide, aluminum metal, and a transition metal oxyanion dissolved in the melt.

Other objects of this invention will become apparent from the following description of the invention.

DETAILED DESCRIPTION OF THE INVENTION The paraffin (alkane) feed to the process of the invention preferably has from two to 12 carbon atoms and is straight chain or branched chain, straight chain paraffins being somewhat more preferred. More preferably, the paraffin has from three to six carbon atoms and the most preferred paraffin is propane.

The molecular oxygen is introduced along with the paraffin feed and is preferably diluted with an inert gas such as nitrogen. It has been found that the optimum hydrocarbon to oxygen ratio is between about l.8:1 and 5.0:]. In general, the volume ratio of oxygen to nitrogen can vary from about 1:2 to 1:6 but preferably it is about 1:4 since this provides a safety factor relative to the explosive limits of the hydrocarbon-oxygen mixture. Air has been found to be a satisfactory source of molecular oxygen. The paraffin and molecular oxygen are continuously passed through a reactor containing molten alkali metal hydroxide. The preferred alkali metal hydroxide is sodium hydroxide.

Aluminum metal is preferably included in the reactor, most preferably in the molten alkali metal hydroxide melt, and is preferably in the shape of small rings or irregular shapes in order to provide reaction surface while at the same time providing a packed condition which allows the free passage of gases therethru.

The amount of aluminum need only be rather small, in general about I to 5 grams of aluminum per 100 ml.

of the molten sodium hydroxide is sufficient but larger quantities can be employed within the scope of the invention.

The reaction may be carried out at temperatures ranging from about 390 C. to about 600 C. The preferred reaction temperature is between about 425 C. and 500 C. The most preferred range is between about 450 C. and 490 C.

Atmospheric pressure is preferred although higher pressures may be used consistent with flammability limits,in general, less than p.s.i.

Incorporated in the alkali metal hydroxide melt in accordance with my invention is a soluble transition metal oxyanion co-catalyst which has been found to cause an increase in the rate of the oxidative dehydrogenation reaction as well as an improvement in selectivity to the desired olefin. The transition metal oxyanion is introduced into the melt as an alkali metal salt, having the formula [Q,M,0,,] wherein z is the valance of the transition metal oxyanion, M is the transition metal which is selected from Group III B thru VIII of the Periodic Table, x and y are the number of atoms of M and 0 respectively in the anion, and w is a number from I to 6. The transition metal oxyanion cocatalyst thus has the formula M 0," wherein M, x, y, and z are as defined above. In the above formula, 1: has a value of l or 2, y has a value of 3 to 7, and z has a value of l to 3. Any compounds fitting this formula which are soluble in the alkali metal hydroxide melt are suitable. The alkali metal can be sodium, lithium, potassium, rubidium, or cesium. The preferred transition metal oxyansions are the dichromate, chromate, molybdate, tungstate, manganate, permanganate, ferrate and metavanadate. Less preferred, but within the scope of the invention, are polymolybdates, polytungstates, and polyvanadates.

Preferably, the alkali metal salt of the above formula is dissolved in the alkali metal hydroxide melt, and is present in a weight ratio range of from about 0.01 to 5 percent based upon the weight of the alkali metal hydroxide. It is most preferable that the compound be completely dissolved, and the solubility limits for each compound at varying temperatures are routinely determinable.

The gaseous hourly space velocity, i.e., the volumes of gaseous feed per volume of molten sodium hydroxide per hour, is preferably from about 50 to 800 and preferably from 100 to 600.

In the following examples runs were carried out utilizing a vertical tubular reactor composed of high purity alumina which measured about 40 inches in length by 1.5 inches in outside diameter. The reactor is provided with a concentric ceramic feed tube about A inch outside diameter which extended to the bottom of the reactor and was provided with apertures at the bottom of the tube to provide a distribution means for the gaseous charge. The gaseous charge was passed downwardly through the feed tube and upwardly through a bed located in the annular space between the feed tube and the inner wall of the reactor. The bed consisted of a bottom layer of aluminum metal rings obtained by cutting an aluminum tube about 3/16 inch outside diameter by 56 inch inside diameter to about /4 inch ring lengths. Above the rings there was provided a layer of tabular alumina (8-14 mesh). When activated alumina was used, the aluminum rings were replaced by the activated alumina. In this reactor 100 ml of molten caustic filled the space between the rings and between the particles of packing and extended upwardly in the tube in the annular space. The co-catalyst is incorporated with the'aluminum and the molten caustic. In some instances sufficient packing was utilized so that thepacking extended above the layer of the molten caustic while in otherruns smaller amounts of packing were used so that the molten caustic layer was above the top of the packing layer. The outside of the reactor tube was surrounded with three heaters so that the temperature of the reaction could be controlled uniformly to the desired level throughout the reaction zone. The top of the reactor tube is provided with conventional fittings to remove the reaction products. In the runs shown in the following examples a reaction temperature of 490 C. was utilized in order to make comparisons of the other variables. Runs have been made in the broad temperature range and in the preferred temperature range with the preferred and most preferred temperatures giving the best results.

It has been found that there is an induction period required to start up the reaction. The fresh reactor assembly is pre-oxidized by passing oxygen through the assembly for several hours, generally overnight or about 16 hours. Thereafter, the feed gas, consisting of the, hydrocarbon, oxygen and nitrogen, is passed through the reactor for several additional hours before high conversions are attained.

in the following examples, runs are shown utilizing the apparatusdescribed. These runs illustrate specific embodiments of the invention and show the preferred conditions for carrying out the reaction of this invention, and should not be construed as limiting the invention.

EXAMPLE I Runs A and B are comparative and not within the present invention since no co-catalyst was used. Run C is within the invention since there is further incorporated in thesodium hydroxide melt 0.1 weight per cent of sodium metavanadate based on the weight of the sodium hydroxide. Run A was carried out at a gaseous hourly space velocity of 101 and a conversion of 12.1..The selectivity to propylene was 79.2 and the yield was 9.7. Run B was carried out at a gaseous hourly space velocity of 102 with a conversion of 11.4, selectivity to propylene of 78.3, and a yield of 9.0. Run C was carried out with the co-catalyst of the invention incorporated in the reactor under otherwise the same conditions except that the gaseous hourly space velocity was adjusted so as to provide equivalent conversion level. In Run C the gaseous hourly space velocity was 108 with a conversion of 20.9, a selectivity to propylene of 79.8, and a yield of 16.6. Thus it can be seen from a comparison of these runs that incorporation of 0.1 weight percent sodium vanadate at approximately equal gaseous hourly space velocity results in much higher conversi%%q:&i elecuv|t1es.

The conditions of Example I were repeated. Run D is comparative, that is, without the incorporation of sodium vanadate and had a gaseous hourly space velocity of 34 with a conversion of 30.9, selectivity of 63.0, and a yield of 19.5. Run E is within the limits .of the invention and is the same as Example. D except that 0.1 weight percent sodium vanadate was incorporated in the melt. The gaseous hourly space velocity was 72 while maintaining a conversion of 30.4, approximately equivalent to the conversion in Run D. Selectivityincreased slightly to 65.0 and yield to 19.8. This example demonstrated that at twice the throughput (space velocity), equivalent conversions, selectivities, and yields can be obtained using the co-catalyst of the invention. That is, the use of this co-catalyst system essentially doubles the reaction rate.

While I have described my invention with great detail, various modifications, improvements, and variations should become readily apparent without departing from the spirit and scope thereof.

1 claim:

1. A method for the oxidative dehydrogenation of alkanes having from two to 12 carbon atoms which comprises reacting said alkane at a temperature ranging between about 390 C. and about 600 C. with molecular oxygen with a volume ratio of alkane to oxygen between about 1.8:1 and 50:1 in the presence of molten alkali metal hydroxide, aluminum or activated alumina, and a compound having the formula [Q,M,O,,],, wherein Q is an alkali metal, M is a transition metal selected from Group 111B to VIII of the Periodic Table, xislor2,yis3to7,wis1to6andzis1to3,said compound being dissolved in said molten alkali metal hydroxide.

2. The method of claim 1 wherein the alkali metal hydroxide is sodium hydroxide and the temperature is in the range of 425 to 500 C.

3. The method of claim 1 wherein the alkanes contain from three to six carbon atoms.

4. The method of claim 1 wherein the alkane is propane.

5. The method of claim 1 wherein Q is sodium.

6.The method of claim 1 wherein said compound is selected from the group consisting of sodium dichromate, sodium molybdate, sodium tungstate, sodium permanganate, and sodium metavanadate.

7. The method of claim 1 wherein said compound is present in the weight ratio range of from about 0.01 percent to about 5 percent, based upon alkali metal hydroxide. 

2. The method of claim 1 wherein the alkali metal hydroxide is sodium hydroxide and the temperature is in the range of 425* to 500* C.
 3. The method of claim 1 wherein the alkanes contain from three to six carbon atoms.
 4. The method of claim 1 wherein the alkane is propane.
 5. The method of claim 1 wherein Q is sodium.
 6. The method of claim 1 wherein said compound is selected from the group consisting of sodium dichromate, sodium molybdate, sodium tungstate, sodium permanganate, and sodium metavanadate.
 7. The method of claim 1 wherein said compound is present in the weight ratio range of from about 0.01 percent to about 5 percent, based upon alkali metal hydroxide. 